ADHD is usually diagnosed in early childhood, before the age of 7, according to the Mayo Clinic. A child younger than 5 may exhibit the characteristics of ADHD beginning as early as infancy. The key indicators of ADHD in a young child are inattentiveness, hyperactivity, impulsiveness or a combination of these three characteristics that last for six months or more and are more pronounced and disruptive than these characteristics in normal children of the same age. The symptoms must also be present under many circumstances. For example, a child who has trouble at preschool or day care but does not exhibit these symptoms at home or during playtime with friends probably does not have ADHD.

Inattentiveness

Children who exhibit inattentiveness often have difficulty paying attention to tasks and tend to make careless mistakes as a result. These kids may leave projects unfinished and become distracted easily, even during play. Children with ADHD may forget things more easily than others their age and frequently lose things, according to the American Association of Family Physicians. They may seem to be not paying attention even when someone is speaking directly to them or giving them instructions and they may have difficulty processing directions or understanding new things. Young children who have ADHD often switch activities unexpectedly and some, especially girls with ADHD, may frequently daydream. Some children exhibit only the symptoms of inattentiveness, not hyperactivity or impulsiveness, and these children may remain undiagnosed if their behavior does not cause obvious problems, explains the National Institute of Mental Health.

Hyperactivity

Hyperactivity in children younger than 5 with ADHD is characterized by constant motion and talkativeness. These kids may have extreme difficulty sitting quietly or completing calm tasks that require concentration. They often dash around and try to touch everything or play with things that are inappropriate. Parents of a young child with ADHD may notice that their child has particular trouble participating in family activities that require sitting or relaxing, such as at dinner time or when being read a story. They also often have difficulty playing by themselves quietly. Kids with ADHD often fidget and squirm when forced to participate in calm activities. Boys with ADHD are more likely to exhibit hyperactivity than girls with ADHD, although most children of both genders with the disorder will exhibit characteristics of hyperactivity, impulsivity and inattentiveness, explains the Mayo Clinic.

Impulsivity

Impulsivity goes hand-in-hand with hyperactivity in children with ADHD, according to the National Institute of Mental Health. Kids who exhibit impulsivity will often blurt out answers to questions before the questioner has finished asking. They may have difficulty controlling their emotions and are often impatient. Impulsive children may interrupt others when speaking and may have difficulty waiting their turn, instead barging their way into other children's play and pushing to the head of the line when told to wait. Children who exhibit impulsivity may have it along with both hyperactivity and inattentiveness or with hyperactivity alone.